WO 2008/140378 discloses a centrifugal separator initially defined for purifying a fluid from contaminating particles. The particles separated from the fluid deposit themselves on the inside of the rotor body in the form of a layer of sludge, wherein the screw conveyor is arranged for transporting the sludge towards and out of the outlet. However, this layer of sludge may be difficult to transport due to the viscosity of the sludge (the viscosity may be too high or low for good transportation characteristics). Furthermore, when rotating the rotor body at high speed the sludge transportation problem may be worsened. The resulting high centrifugal forces have a compressing effect on the sludge making it more difficult to transport out of the sludge outlet. Failure to discharge the sludge from the rotor body will cause a relatively solid sludge phase to grow radially inwards towards the axis of rotation, impairing the degree of separation and ultimately rendering continued separation impossible because of obstruction.